Whales are the biggest animals in the world. Why are they only found in the
ocean?

If you walk into the ocean, or any deep water, you'll notice that you begin
to float, or at least become neutrally bouyant. All mammals need to be able
to move around to find food and for many other reasons. A big animal on the
land, such as an elephant, needs to have huge legs and must spend
tremendous energy just to move around. In the water however, gravity has
virtually no effect on a mammal, so movement becomes much more graceful and
requires less energy. Thus an animal that lives in the ocean can become
much bigger. Still, it's not impossible for land animals to become very
large if conditions are right. Some dinosaurs were nearly as big as the
biggest whales.